With the ever-increasing utilization of electromagnetic waves in broadcasting, mobile communication, radar, cellular phones, wireless LAN and other systems, more electromagnetic waves are scattering in the ambient atmosphere, frequently giving rise to the problems of electromagnetic wave disturbance and electronic equipment malfunction. One approach for overcoming these problems is to attach electromagnetic wave absorbers to the walls of buildings, bridges and structures providing substantial reflection and scattering of electromagnetic waves and has been effective.
This approach addresses electromagnetic wave absorption in far fields (plane waves). The electromagnetic wave absorber is generally constructed from an electromagnetic wave absorber of rubber or plastics uniformly filled with a magnetic-loss material such as ferrite or a dielectric-loss material such as carbon in powder form, and an electromagnetic wave reflector of a metal or the like disposed on the rear surface of the absorbing material.
An electromagnetic wave absorber is attached to a structure in several ways. In the case of a structure whose surface is made of a metal or the like, an electromagnetic wave absorber without an electromagnetic wave reflector is directly attached to the structure while the structure itself is utilized as an electromagnetic wave reflector. Alternatively, an electromagnetic wave absorber is lined with an electromagnetic wave reflector such as metal foil before it is attached to the structure.
The electromagnetic wave absorber is required to be flexible in order to conform to various shapes of structure to which the absorber is attached. However, prior art electromagnetic wave reflectors lack flexibility because they are usually metal foil, metallized film, metal fibers, carbon fiber fabric, and metallized glass fabric. As a solution to this problem, Japanese Patent No. 3,097,343 discloses a flexible thin-wall electromagnetic wave absorber comprising, in lamination, a flexible sheet-shaped electromagnetic wave absorbing layer composed of a mixture of an electromagnetic energy-loss material and a binder, and a radio wave reflecting layer of an organic fiber fabric having a high conductivity metal material electroless plated thereon.
In these applications, the electromagnetic wave absorber is further required to be resistant to weathering when it is used outdoor. To this end, a protective layer is separately provided on the surface of the electromagnetic wave absorbing layer. For long-term service, a firm bond is required between the electromagnetic wave absorbing layer and the electromagnetic wave reflecting layer. However, it is not technically easy to develop an electromagnetic wave absorber which satisfies all of flexibility, weather resistance, and firm bond between electromagnetic wave absorbing and reflecting layers.
Another electromagnetic wave disturbance is electromagnetic wave disturbance in near fields. A progress has been made for the fabrication to a higher density and higher integration of electronic parts such as CPU, MPU and LSI arranged within personal computers, cellular phones and other electronic equipment and the mounting in a higher density of electronic parts on printed circuit boards. Electromagnetic waves are radiated in the interior of equipment and reflected thereby so that the interior is full of electromagnetic waves, and electromagnetic interference can occur with the electromagnetic wave emitted by the equipment itself.
In the prior art, an artisan with specialized knowledge and experience of noise suppression must be engaged in taking a countermeasure against disturbances by electromagnetic interference. It is a time-consuming task to find an effective countermeasure. Another drawback is that an electronic component in question must be previously given an extra space for mounting a shield. To solve these problems, engineers are interested in electromagnetic absorbers which absorb electromagnetic waves for thereby reducing reflected and transmitted waves.
For preventing electromagnetic waves from leaking out of equipment, it has also been practiced to install metallic plates as the electromagnetic wave shield or to impart electric conductivity to housings for endowing them with an electromagnetic wave shielding function. The electromagnetic waves reflected and scattered by the shield are then confined within the equipment interior, raising the problems of enhanced electromagnetic interference. Electromagnetic interference can occur between substrates mounted within the equipment.
To overcome the above problems, JP-A 7-212079 proposes an electromagnetic interference suppressor comprising an electrically conductive support and an insulating soft magnetic layer laminated thereon and formed of a soft magnetic powder and an organic binder.
The current trend toward a higher density and higher integration of CPU, MPU, LSI and other components used in electronic equipment encounters the problem of increased heat release. Ineffective cooling will cause thermal runaway or undesired effects, giving rise to malfunction. One prior art means for effectively radiating heat to the exterior is to dispose heat transfer media such as silicone grease and silicone rubber filled with heat conductive powder between CPU, MPU or LSI and heat sinks for reducing the contact thermal resistance therebetween. This means, however, cannot avoid the problem of electromagnetic interference within the equipment interior.
Therefore, what is required in the electronic equipment interior, especially at sites where electronic components such as CPU, MPU and LSI are integrated in a high density is a member having all the functions of electromagnetic wave absorption, electromagnetic wave shielding and heat transfer. Members satisfying these three functions are unavailable in the art. In addition to these three functions, the application contemplated herein requires flexibility and heat resistance, and in the case of a multilayer structure including electromagnetic wave absorbing and reflecting layers, firm adhesion between the layers is also needed. Members satisfying these functions at the same time are unavailable in the art.